This invention relates to a prism viewfinder for reflex cameras incorporating means for measurement of the exposure, this viewfinder being of the kind in which the prism has a bevel face opposite that at which the incident light enters and a plane mirror face associated with this bevel face and reflecting the light beam received from the latter away from the prism at an inclination of approximately 45.degree. to the direction of original incidence on the prism, in which an eyepiece is disposed adjacent the face of the prism from which the reflected light eventually emerges from the latter, and in which a light-sensitive element forming part of the exposure measuring means or exposure meter is arranged adjacent the eyepiece.
In cases where the prism of a viewfinder of a reflex camera is a five-sided, so-called pentagon, prism, the arrangement is usually so devised that the light-sensitive element of the exposure meter is installed beside the eyepiece. The light-sensitive element then measures the exposure on the plane mirror surface which diverts the light beam to the eyepiece. Since the spacing of the eyepiece and the light-sensitive element from this plane mirror face is comparatively large and the longitudinal dimension of the prism viewfinder corresponds, there has to be a comparatively sharp acute angle between the orientation of the eyepiece and that of the light-sensitive element to enable useful exposure results to be achieved.
The measurements of an exposure meter for built-in exposure metering are useful when the complete image face is assessed approximately uniformly and a situation in which some areas of the image have a different degree of exposure than others is avoided. The pentagon prism previously mentioned is usually so devised that the optical axis of the eyepiece is parallel to that of the camera.
German specification DE-PS No. 12 66 630 describes a prism viewfinder for reflex cameras in which the direction of survey is at an angle of 45.degree. to the base face of the prism or the optical axis of the camera. Inter alia this has the advantage that when a stand which is not too high is used the head can have a comfortable and favourable position. A prism viewfinder of this character has a bevel surface opposite the face at which the light beam enters and a plane mirror surface associated with this and deflecting the beam from the prism in the required fashion.
German specification DE-PS No. 20 21 179 discloses the arrangement of a light-sensitive element beside the eyepiece at the light emergent face of the prism and a built-in exposure meter. A prism viewfinder of this type incurs however the following difficulty. The distance between the eyepiece, or the beam emergent face, and the plane mirror surface is substantially less than in the case of the previously-mentioned pentagon prism. The dimensions of the eyepiece mean that the light-sensitive element is no longer in a position to measure the exposure at the plane mirror surface to any significant degree, and in addition the light-sensitive element receives light beams diffracted from different parts of the prism in a manner which is difficult to control. The integral exposure value thus is quite incorrect and much depends on how the bright and dark surfaces of any particular object are distributed.
A special problem with prism viewfinders of the type first set forth above in which the light passage is deflected out of the prism at approximately 45.degree. inclination to its incident direction, lies in the fact that the outer dimensions of this prism are comparatively large and it is therefore inescapably necessary, during the arrangement of the exposure meter, to take care that the outer dimensions of such a prism viewfinder are not further increased.